This application claims priority to an application entitled xe2x80x9cData Transmission Apparatus and Method for an HARQ Data Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on May 22, 2000 and assigned Serial No. 2000-28477, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a data transmission apparatus and method in a radio communication system, and in particular, to an apparatus and method for managing retransmission of data which is subjected to transmission error during data transmission.
2. Description of the Related Art
In a radio communication system, linear block codes such as convolutional codes and turbo codes, for which a single decoder is used, are chiefly used for channel coding. Meanwhile, such a radio communication system employs an HARQ (Hybrid Automatic Repeat Request) Type I using the ARQ (Automatic Repeat Request) scheme which requires retransmission of data packets upon detection of an FEC (Forward Error Correction) code and an error. The radio communication system includes a satellite system, an ISDN (Integrated Services Digital Network) system, a digital cellular system, a CDMA-2000 (Code Division Multiple Access-2000) system, a UMTS (Universal Mobile Telecommunication System) system and an IMT-2000 (International Mobile Telecommunication-2000) system, and the FEC code includes the convolutional code and the turbo code.
The above-stated hybrid ARQ scheme is generally divided into HARQ Type I, HARQ Type II and HARQ Type III. At present, most of the multi-access schemes and the multi-channel schemes using the convolutional codes or the turbo codes employ the HARQ Type I. That is, the multi-access and multi-channel schemes of the radio communication system using the above-stated channel coding scheme, employ the HARQ Type I as an ARQ scheme for increasing the data transmission efficiency, i.e., throughput of the channel coding scheme and improving the system performance.
A principle of the first ARQ scheme is based on the fact that the channel encoder using the convolutional code, the turbo code or the linear block code has a constant code rate. FIGS. 1A and 1B illustrate a conceptual data process flow by the HARQ Type I.
Commonly, a transmitter of a radio communication system combines L-bit transmission data with a CRC (Cyclic Redundancy Check) code for error correction and then codes the combined data, L+CRC, through channel coding. The transmitter performs a separate processing process on the coded data, (L+CRC)xc3x97Rxe2x88x921, and then, transmits the processed data through an assigned channel. Meanwhile, a receiver of the radio communication system acquires the original L-bit data and the CRC code through a reverse operation of the transmitter, and transmits a response signal ACK/NAK to the transmitter according to the CRC check results.
This will be described in more detail with reference to FIG. 1A. a CRC encoder 110 receives an L-bit source data packet and encodes the received data using a CRC code, creating a coded data block, L+CRC. Commonly, CRC bits are added to the input data before channel encoding. A channel encoder 112 performs channel coding on the coded data block, L+CRC, creating a channel-coded data block, (L+CRC)xc3x97Rxe2x88x921. The channel-coded data block (L+CRC)xc3x97Rxe2x88x921, is provided to a specific channel through other function blocks 114 necessary for multiplexing.
Other inverse function blocks 116 necessary for demultiplexing in a receiver receiving the coded data block through the specific channel, demultiplex the received coded data block and output a received channel-coded data block, (L+CRC)xc3x97Rxe2x88x921. A channel decoder 118 then performs channel decoding on the received channel-coded data block, (L+CRC)xc3x97Rxe2x88x921, and outputs a channel-decoded data block, L+CRC. A CRC decoder 120 performs CRC checking on the channel-decoded data block, L+CRC, to acquire the original data, i.e., the L-bit source data packet. After completion of CRC checking, the CRC decoder 120 performs CRC checking using the CRC decoding results, thereby to determine whether the source data packet has transmission errors.
If no error is detected through the CRC check, the receiver provides the source data packet to an upper layer and transmits a confirm signal ACK (Acknowledgement) acknowledging the source data packet to the transmitter. However, upon detecting an error through the CRC check, the receiver transmits a confirm signal NAK (Not-Acknowledgement) requesting retransmission of the source data packet to the transmitter.
After transmitting the channel-coded data block, the transmitter receives the confirm signal ACK/NAK from the receiver in response to the transmitted data block. Upon receipt of the confirm signal NAK, the transmitter retransmits the corresponding data block in the above-described operation. The transmission scheme includes Stop-and-Wait ARQ, Go-Back-N ARQ, and Selective-Repeat ARQ schemes. The detailed description of the retransmission schemes will be omitted.
FIG. 1B illustrates a conceptional transmission procedure of the source data packet between the transmitter and the receiver. In FIG. 1B, the transmitter retransmits the coded data block upon every receipt of m NAKs from the receiver.
As an example of such a procedure, in an air interface of the 3GPP-2 (3rd Generation Project Partnership-2; a standard for a synchronous CDMA system) mobile communication system (hereinafter, referred to as xe2x80x9cCDMA-2000xe2x80x9d system), the multi-access scheme and the multi-channel scheme of the system employ the HARQ Type I in order to increase data transmission efficiency of the channel coding scheme and to improve the system performance. In addition, in an air interface of the 3GPP (3rd Generation Project Partnership; a standard for an asynchronous CDMA system) mobile communication system (hereinafter, referred to as xe2x80x9cUMTS systemxe2x80x9d) the multi-access scheme and the multi-channel scheme of the system employ the HARQ Type I in order to increase data transmission efficiency of the channel coding scheme and to improve the system performance.
However, the HARQ Type I has the following disadvantages.
First, the HARQ Type I has higher throughput, compared with a pure ARQ scheme. However, as a signal-to-noise ratio (S/N) of a signal is increased more and more, the throughput becomes saturated to a code rate R of the FEC code, thus resulting in a reduction in the throughput as compared with the pure ARQ. That is, the throughput cannot approach to 1.0 (100%) even at a very high S/N. Such a problem is shown by a characteristic curve of the HARQ Type I in FIG. 2. That is, as for the HARQ Type I, the throughput is saturated to the code rate R ( less than 1.0) as shown in FIG. 2, so that it cannot approach to 1.0.
Second, the HARQ Type I improves the throughput by performing error correction using the FEC code, compared with the pure ARQ. However, since the HARQ Type I uses a constant redundancy, i.e., constant code rate regardless of a variation in S/N, it has low transmission efficiency. Therefore, the HARQ Type I cannot adaptively copes with variation of the channel condition, thus causing a decrease in the data rate.
To solve theses problems, the HARQ Type II and the HARQ Type III are used. The HARQ Type II and the HARQ Type III have an adaptive structure which adaptively determines an amount of the redundancy used for the FEC code according to how good the channel condition is. Therefore, the HARQ Type II and the HARQ Type III have the improved throughput, compared with the HARQ Type I. That is, the adaptive structure reduces the amount of the redundancy to the minimum, so that as S/N of the signal is increased more and more, the code rate R of the FEC code approaches to 1, thereby enabling the throughput to approach to 1. Meanwhile, the adaptive structure performs optimal error correction such that if SIN of the signal is decreased, the amount of the redundancy is increased to the maximum to enable the code rate R of the FEC code to approach to 0, or the redundancy is repeated so as not to enable the throughput to approach to 0. Accordingly, the HARQ Type II and the HARQ Type III have the improved throughput at both the low S/N and the high S/N.
Here, a difference between the HARQ Type II and the HARW Type III is as follows.
The HARQ Type II sets an initial code rate RI to 1 or a value slightly less than 1 before transmitting the data block, and thereafter, retransmits only the redundancy whose code rate is always higher than 1. Therefore, the HARQ Type II cannot perform decoding using only the second transmitted redundancy or the third transmitted redundancy, and should perform decoding by combining the previously transmitted data block (or redundancy). On the other hand, the HARQ Type III sets the initial code rate RI to a value lower than 1 before transmitting the data block, and even thereafter, transmits the redundancy whose code rate is lower than 1. Therefore, the HARQ Type III can perform decoding using only the second transmitted redundancy or the third transmitted redundancy respectively.
However, compared with the HARQ Type II, the HARQ Type III has the low throughput in a good channel condition. In addition, a code structure used in the HARQ Type III includes a complementary code. However, the HARQ Type III does not always use this code, and can also use a given code whose code rate is higher than 1.
Meanwhile, what is most important in the HARQ Type II and the HARQ Type III is to determine a size of first transmitted coded data block for one input data block (hereinafter, referred to as xe2x80x9csource data packetxe2x80x9d) to be transmitted, its associated code rate and coding scheme, and determine a size of a coded data block used during each retransmission, its associated code rate and coding scheme. For example, assuming that mother code of an original channel encoder has a code rate R=⅓ and the system can retransmit each coded data block three times, the code rate result for each retransmission can be determined as shown in Table 1 below.
The second redundancy version has code rate xc2xd in the table 1, but it means making the code rate xc2xd by first and second redundancy transmission. And the third redundancy version has code rate ⅓ in the table 1, but it means making the code rate ⅓ by first, second and third redundancy transmission. Therefore, the code rate of each transmission can be same.
Even when the code rate for each retransmission is determined as shown in Table 1, there are various ways to determine which one of the redundancy bits derived from the mother code corresponding to the respective code rates is to be transmitted during the second retransmission and which one of the redundancy bits is to be transmitted during the third retransmission. In some cases, there is a great difference between the deteriorated performance and the improved performance, caused by the selected redundancy bits. Therefore, selecting the redundancy bits guaranteeing the optimal performance is a very important factor.
However, there has been proposed no concrete design rule for the case where the multi-access scheme and the multi-channel scheme of the 3GPP-2 CDMA-2000 system including the existing data communication system employ the channel coding scheme, or where the multi-access scheme and the multi-channel scheme of the 3GPP UMTS system employ the HARQ Type II and the HARQ Type III. That is, a rare research has been carried out on the HARQ Type II and the HARQ Type III for providing the optimal performance by combining the convolutional code or the turbo code with the ARQ scheme, for the systems using the multi-access scheme and the multi-channel scheme.
In particular, regarding the air interface standard for the 3GPP-2 CDMA-2000 system, research has been carried out on the application of the HARQ Type II and the HARQ Type III to increase the data transmission efficiency at the data transmission channel and to improve the system performance. This technical field is related to the FEC code and the ARQ scheme, which are closely connected with an increase in reliability and an improvement in the throughput of the digital communication system. That is, this field is related to performance improvement of the next generation system as well as the existing digital communication system.
The HARQ Type II and the HARQ Type III used by the current data communication systems must be constructed to reflect the following conditions in order to resolve the performance problems and guarantee the optimal performance. Generally, in the multi-access scheme and the multi-channel scheme of the system employing a channel encoder which uses the convolutional codes and the turbo codes or the linear block codes for channel coding, a variable rate transmission scheme is typically used to increase data transmission efficiency of the channel coding scheme and improve the system performance. In this case, symbol puncturing or symbol repetition is generally used.
The following conditions should be duly considered and reflected in order to guarantee performance of the FEC code.
First, the coded symbols output from the encoder are punctured using a uniform puncturing pattern, i.e., a periodic pattern, if possible, and the period (or cycle) of the puncturing pattern should be minimized. Second, the number of puncturing bits should be minimized, if possible. Third, the coded symbols output from the encoder are repeated using a uniform repetition pattern, i.e., a periodic pattern, if possible, and the period of the repetition pattern should be minimized. Finally, the number of the repetition bits should be maximized, if possible.
In addition, concatenated codes such as turbo codes using reiterative decoding may have the following disadvantages. Determining to which component decoder of the reiterative decoder the redundancy transmitted during each retransmission belongs is a very important factor in determining performance of the FEC code. Retransmission of the redundancy should be performed considering this.
As mentioned above, the conventional data communication system has the following disadvantages.
First, there has been proposed no concrete design rule for the case where the multi-access scheme and the multi-channel scheme of the CDMA-2000 system including the conventional data communication system employ the channel coding scheme, or where the multi-access scheme and the multi-channel scheme of the UMTS system employ the HARQ Type II and the HARQ Type III.
Second, what is most important in the HARQ Type III and the HARQ Type III is to determine a size of first transmitted coded data block for a source data packet, its associated code rate and coding scheme, and determine a size of a coded data block used during each retransmission, its associated code rate and coding scheme. However, the conventional data communication system is not provided with a rule for determining the code rate.
Third, the HARQ Type III and the HARQ Type III generally employ symbol puncturing or symbol repetition for redundancy selection. In this case, the above-mentioned conditions should be duly considered and reflected to guarantee performance of the FEC code. However, such conditions are not specifically reflected in the existing technology.
Fourth, the conventional HARQ Type II and HARQ Type III employs a method for basically regarding the whole codes as one unit and separating the redundancy from this, from the viewpoint of the system using the FEC code used for a single decoder. However, this should be differently interpreted in the case of the FEC code using the iterative decoding such as the turbo codes. That is, the redundancy should be selected to be optimal to the decoding method of the iterative decoder. The redundancy should not be separated simply from the viewpoint of the encoder.
It is, therefore, an object of the present invention to provide an apparatus and method for most efficiently embodying the conditions necessary for HARQ Type II and HARQ Type III.
It is another object of the present invention to provide a hybrid ARQ scheme for improving performance of a radio communication system through efficient combination of a channel coding scheme and an ARQ scheme in a multi-access scheme and a multi-channel scheme of the system.
It is further another object of the present invention to provide a hybrid ARQ scheme showing optimal performance in a radio communication system using convolutional codes, turbo codes or linear block codes.
It is still another object of the present invention to provide a method for determining a size of a first transmitted data block for a source data packet, its associated code rate and code, and also determining a size of a data block used for retransmission, its associated code rate and coding scheme.
To achieve the above and other objects, there is provided a method for transmitting a coded data, L information bits and sequences of parity bits, in an HARQ transmission system including a turbo encoder for receiving L input information bits and generating the L information bits and M (xe2x89xa72) sequences of L parity bits for the information bits. The method comprises transmitting the L information bits and part of sequences of the parity bits determined by one of two integers closer to (N1xe2x88x92L)/M where N1 indicates a number of transmission bits given when an initial transmission code rate of the turbo encoder is below 1 during initial transmission; and at a receiver""s retransmission request due to failure to receive the information bits transmitted during the initial transmission, transmitting sequences of parity bits determined by one of two integers closer to N2/M where N2 indicates a number of the transmission bits given when a retransmission code rate of the turbo encoder is below 1.
Further, the system transmits the L information bits when the initial transmission code rate of the turbo encoder is 1 during initial transmission, and transmits part of sequence of L parity bits determined by adding up L/M parity bits provided from said each sequence of parity bits at a receiver""s retransmission request due to failure to receive the information bits transmitted during the initial transmission.
Preferably, the initial transmission code rate of the turbo encoder during the initial transmission is determined by predetermined maximum throughput of the turbo encoder.